This invention relates generally to phase-locked loop frequency synthesizers, and more particularly to wide-band phase-locked loop frequency synthesizers that provide a flat modulation response.
In frequency synthesizers having voltage controlled oscillators (VCOs), obtaining a flat modulation response is difficult because of non-linear characteristics of the components comprising the VCO. Conventional VCOs typically utilize voltage variable capacitors commonly known as varactor diodes in their implementations. Normally these varactor diodes are arranged to parallel resonate with an inductor in a so-called "tank circuit", such that as the voltage (control voltage) applied to the varactor diodes varies, their capacitance change resulting in a shift in the resonant frequency of the oscillator. As a result, the oscillator frequency varies directly with the control voltage, albeit in a non-linear fashion. Described alternately, the higher the control voltage, the higher the oscillator frequency. Conversely, the lower the control voltage, the lower the oscillator frequency. In addition to oscillator frequency, the VCO's gain factor K.sub.o (expressed in MHz/volt) varies inversely with the control voltage, but also in a non-linear fashion. In other words, the VCO has a higher gain factor K.sub.o at lower control voltages, and a lower gain factor K.sub.o at higher voltages.
In a modulation system for use in a radio transmitter, the control voltage of the VCO is typically modulated with information signals, such as audio frequency signals to produce a resultant frequency modulated (FM) signal having the information signals appear spectrally as frequency deviations from a radio frequency carrier. However, to parallel resonate a fixed inductor over a frequency band to achieve constant deviation, a smaller variation in capacitance is required at the higher frequencies, while a greater variation in capacitance is required at the lower frequencies. Since the capacitance varies directly with the square root of the varactor diode voltage, at the lower frequencies, where a low level of control voltage is utilized, modulation may be quite high (because the voltage provided is not enough to vary the capacitance needed and the gain factor K.sub.o is too high for a constant FM deviation). However, at the higher frequencies where the control voltage must be at a higher level, the resultant modulation falls off substantially (because the voltage provided is too large, creating a change in capacitance and a gain factor K.sub.o that are too high to result in the same constant FM deviation).
The failure to provide constant modulation may cause a radio to fail to meet government regulations. Normally, government regulations specify a constant level of maximum modulation from a transmitter at each frequency at which the transmitter is operated.
As illustrated in U.S. Pat. No. 4,649,353, one approach to "flattening" the modulation response utilizes an adaptive charge-pump at the input to the loop filter to compensate for the non-constant modulation (where a loop filter in a conventional frequency synthesizer filters the control voltage provided by a phase detector before feeding the control voltage into the VCO). For this approach, the preferred embodiment utlizes three transistors to reciprocally compensate for the nonlinear gain factor K.sub.o.
Another approach, illustrated in U.S. Pat. No. 4,510,465, utilizes an audio compensator that uses the control voltage at the input to the loop filter to couple an output signal into the VCO. A single transistor is utilized to provide for a substantially constant gain factor K.sub.o.